Determining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics

Abstract

Abstract. The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be∕26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83 +17/-5 m Ma−1 during the last ca. 4 Myr (Pliocene–Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene–Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate–orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation.

Weathering Incongruence in Mountainous Mediterranean Climates Recorded by Stream Lithium Isotope Ratios

“…Seismic surveys show that the weathering profile extends as deep as 10 m below the land surface (Dangeard et al, 2019). Regional incision rates over the last 4 Ma based on cosmogenic isotopes are 83 +17/-5 m/Ma (Malcles et al, 2020), which is consistent with vertical velocities of <0.5 mm/yr estimated from Global Positioning System stations in the surrounding area (Masson et al, 2019).…”

Denudation history and palaeogeography of the Pyrenees and their peripheral basins: an 84-million-year geomorphological perspective

“…Mediterranean extension appear to have played a subsidiary and progressively declining role in the rise of the Modern Pyrenees. Uplift, which has impacted not just the mountain range but also its foreland basins and perhaps the entire Iberian Plate, and likewise the Massif Central (Macles et al, 2020) is now attributed to subcrustal and sublithospheric processes driven by boundary conditions that are only partly related to the intrinsic metabolism of an orogenic crustal wedge (see Section 2.4 and Section 4.2.1.1; Pous et al, 1995a, 1995b, Lewis et al, 2000Barruol et al, 2002Barruol et al, , 2004Gunnell et al, 2008Gunnell et al, , 2009Boschi et al, 2010;Casas-Sainz et al, 2009;Chevrot et al, 2014Chevrot et al, , 2015Chevrot et al, , 2018Dufréchoux et al, 2018;Wehr et al, 2018;Conway-Jones et al, 2019;Ortiz et al, 2020;Jolivet et al, 2020). Still unexplained is the apparent acceleration of the uplift and its unsteady regime, illustrated for example by the contrast between generation P2 of wide pediments penetrating deep into parts of the orogen, and the abrupt vertical incision of these planar landforms by the deep, often Vshaped, younger valleys.…”

Investigation of stochastic-threshold incision models across a climatic and morphological gradient

“…Paleofloods and historical floods have been documented in our study area, in particular along the Ardèche river (Sheffer et al, 2003;Naulet et al, 2005) and the Gardon river (Dezileau et al, 2014), allowing the assumption of a high-discharge-variability regime since at least the middle Holocene. The absolute values of precipitation and discharge variability have probably changed through time and been modulated by the succession of glacial cycles, but, as the present-day relief is associated with late Miocene to Pliocene uplift events (Olivetti et al, 2016(Olivetti et al, , 2020Malcles et al, 2020), it can be assumed that the orographic pattern and associated relative climatic contrasts across the margin are persistent throughout the Quaternary.…”

“…Due to their connection to the Rhône valley the eastern drainage systems were probably affected by the Messinian salinity crisis (Mocochain et al, 2009;Tassy et al, 2013). The Cenozoic uplift history of the Massif Central and its mechanisms are still poorly documented, but it is suspected that most of the observed relief, with respect to the Rhône valley, is associated with distinct late Miocene to Pliocene uplift events (Olivetti et al, 2016;Malcles et al, 2020). The present tectonic activity of the Massif Central is limited, with a recent uplift rate < 100 mm kyr −1 .…”